The present invention relates to fluid coupling devices such as rotating unions and, more particularly, to an improved seal design that permits a coupling device, having a single inlet passageway and latching external means of lubricating the sealing surfaces used therein, to be operated with lubricating media, such as water-based or oil-based coolants, with non-lubricating media, such as dry air and gases, and with no media of any kind.
Rotating unions are known alternately as fluid couplings, coolant unions, or rotary joints and are employed in high-speed drilling and boring transfer operations, in high-speed machine tool spindles and in other operations where it is necessary to transfer a fluid medium to a rotating device. To optimize the operation of high-speed machine tools, rotating unions are required to conduct either a liquid, such as a water-based or an oil-based coolant, or a dry gas, such as air, from a source of the particular coolant to the rotating machine tool. One such application of a rotating union is the delivery of a liquid to the rotating machine tool as a coolant and lubricant and the subsequent delivery of air to the rotating machine tool for cleaning the machine tool or work area. Another application of a rotating union involves the high-speed machining of special heat-treated steels, wherein the cutting tool may become so hot that a water-based or oil-based coolant would result in thermal shock of the cutting tool. Such applications warrant the use of air-based coolants to cool the cutting tool. A third application of a rotating union involves the machining of certain medical components, where contact with a lubricating medium would render the finished part unfit for use.
When a rotating union must conduct a lubricating medium, one of a number of different arrangements may be employed to ensure tight engagement of the rotating and non-rotating seal members. However, in all such arrangements, the interfacing surfaces of the seal members must be lubricated to avoid a condition known in the art as “dry running”. Known prior art, such as U.S. Pat. Nos. 6,149,160, 6,325,380 and 6,726,213, describe a number of treatments of the face seals which promote lubrication of the interfacing seal surfaces. The running condition results in increased wear on the interfacing seal surfaces, particularly at high rotational speeds, and extended periods of dry running operation will cause severe damage to the seal members, thereby requiring replacement of some or all of the rotating union.
When a rotating union must operate in the absence of any medium, the dry running condition is typically prevented by automatically separating the rotating and non-rotating seal surfaces. Such arrangements are commonly referred to as “pop off” designs. In such designs, the non-rotating seal surface is mounted on a carrier that moves axially in relation to the rotating seal surface and engages the rotating seal surface in the presence of a medium and disengages the rotating seal surface in the absence of a medium. However, single-inlet “pop off” rotating unions have heretofore had the disadvantage of engaging the seal surfaces in the presence of both lubricating and non-lubricating media.
When a rotating union must conduct a non-lubricating medium may be employed to separate the interfacing seal surfaces by a microscopic amount to prevent day running and to insure that the interfacing surfaces are not in direct contact. U.S. Pat. Nos. 5,669,636 and 6,406,065 are examples of rotary unions that require at least two separate inlets to operate with lubricating and non-lubricating media. Such arrangements have the disadvantage of requiring complicated and additional piping and valves systems to control the multi-media operations.
Still another arrangement to maintain contact between the interfacing seal surfaces for conducting a non-lubricating medium is described in Japanese application 10-302395 (PCT 2000-130665, for example). To overcome the problem of dry running, such an arrangement attempts to lubricate the interfacing seal surfaces with a separate, externally supplied medium, such as an oil mist. However, such an arrangement has the disadvantage of requiring a separate source of dispensing system for providing the oil mist to the interfacing seal surfaces. This system adds cost and complexity and such arrangements have the further disadvantage of allowing the oil mist to contaminate water-based coolant. This contamination can occur both outside the seals, as a result of excess oil mist running into the rotating union's drain line, and inside the seals, as a result of oil mist being conveyed by so-called “hydropads” or grooves, a concept described in U.S. Pat. No. 5,941,532. Either results in so-called “tramp oil” that must be filtered out from the liquid or water-based coolant, creating further expense in the operation of the rotary union.
As used in the disclosure that follows and as is well known in the art, the term “balance ratio,” B, is defined as the ratio between the average load imposed by the sealed pressure on the seal face of the primary seal assembly of a rotary union, pf, over the sealed pressure, p, which can be generally expressed algebraically as:
  B  =            p      f        p  The average load pf involves an expression of the hydraulic surface area and/or dimensions of hydraulic surfaces that contribute to the action of a hydraulic force on the primary seal assembly. In other words, the balance ratio is a non-dimensional parameter that encompasses the hydraulically significant structures of a seal that yield the net hydraulic force during seal operation. With the foregoing in mind, the net hydraulic force tending to engage or disengage a face seal is the result of a balancing between forces acting on the seal in an opening direction and forces acting on the seal in a closing direction. When the opening and closing forces are balanced, the seal will remain stationary even when pressurized. However, when either the opening or closing force is greater than the other, the seal carrier will be urged to move.